The abundant-centre hypothesis (ACH) predicts that the density of a species should peak at its distribution centre and decrease similarly towards distribution margins. The ACH has been deduced from a theory that postulates that environmental conditions should be most favourable for a species at the centre of its distribution. This idealized density pattern, however, has been supported by limited field studies, as natural patterns are often more complex. It is thus of interest to examine under

more »

... at conditions compliance with the ACH could be favoured. Such conditions could be smooth environmental gradients with limited habitat patchiness throughout the distribution range of a species. Thus, we tested the ACH by measuring the density of an intertidal barnacle (Semibalanus balanoides) across its full vertical distribution range (from low to high elevations) on a rocky shore with similar substrate properties across elevations. To do a reliable test, we surveyed eight elevation zones applying an equal sampling effort in each zone. Average barnacle density conformed to the ACH, as it peaked at the middle of the vertical distribution range of this species. The same underlying theory predicts a similar unimodal pattern for maximum body size, but this trait was decoupled from density, as maximum barnacle size increased from low to high elevations. Overall, although the ACH is not a universal predictive tool as once envisioned, it may predict some cases well, as shown by this study. Therefore, the ACH should not be discarded completely, and its domain of application should be further evaluated. ABSTRACT 9 The abundant-centre hypothesis (ACH) predicts that the density of a species should peak at 10 its distribution centre and decrease similarly towards distribution margins. The ACH has been 11 deduced from a theory that postulates that environmental conditions should be most favourable 12 for a species at the centre of its distribution. This idealized density pattern, however, has been 13 supported by limited field studies, as natural patterns are often more complex. It is thus of 14 interest to examine under what conditions compliance with the ACH could be favoured. Such 15 conditions could be smooth environmental gradients with limited habitat patchiness throughout 16 the distribution range of a species. Thus, we tested the ACH by measuring the density of an 17 intertidal barnacle (Semibalanus balanoides) across its full vertical distribution range (from low 18 to high elevations) on a rocky shore with similar substrate properties across elevations. To do a 19 reliable test, we surveyed eight elevation zones applying an equal sampling effort in each zone. 20 Average barnacle density conformed to the ACH, as it peaked at the middle of the vertical 21 distribution range of this species. The same underlying theory predicts a similar unimodal pattern 22 for maximum body size, but this trait was decoupled from density, as maximum barnacle size 23 increased from low to high elevations. Overall, although the ACH is not a universal predictive 24 tool as once envisioned, it may predict some cases well, as shown by this study. Therefore, the 25 ACH should not be discarded completely, and its domain of application should be further 26 evaluated. Manuscript to be reviewed 29 The abundant-centre hypothesis (ACH) predicts that the density of a species should peak at 30 its distribution centre and decrease similarly towards the distribution margins. The ACH has 31 been deduced from a theory that postulates that environmental conditions should be most 32 favourable for a species at its distribution centre (Brown, 1984). These concepts were conceived 33 without particularly considering the extent of species distributions, which can be large or small 34 depending mainly on limits imposed by the physical environment. In addition, distribution 35 ranges can be examined in various directions (e.g., horizontally along forest transects or 36 vertically across mountain elevations). The convenient simplicity of the ACH has been used to 37 generate more complex hypotheses predicting, for example, changes in population persistence, 38 genetic diversity, speciation potential, and consumer control from the distribution margins of a 39 species to its distribution centre (Lawton et al.